ST AN1283 Application note

A BATTERY CHARGER USING TSM101

This te ch n i ca l no t e s h ows ho w t o us e t h e TS M 10 1 integrated circuit with a switching mode power supply (SMPS) to realize a battery charger. An example of realization of a 12V Nickel-Cadmium battery charger is given.

1 - TSM101 PRESENTATION

The TSM101 integrated circuit incorporates a high stability series band gap voltage reference, two ORed operational amplifiers and a current source (Figure 1).

AN1283

APPLICATION NOTE

by S. LAFFONT and R. LIOU

A current limitation is used to prot ect the power supply against short circuits, but lacks precision. This limitation is generally realized by sensing the current of the power transistor, in the primary side of th e SMPS.

The role of the TSM101 is to make a fine regulation of the output current of the SMPS and a precise voltage limitation.

The primary current limitation is conserved and acts as a security for a fail-safe operation if a short-circuit occurs at the output of the charger.

Figure 1 : TSM101 Schematic Diagram

1.24V

CSEN

1.4mA

CRREF

GND CRIN

VCCVREF

VRIN

OUT

This IC compares the DC voltage and the current level at the output of a switching power supply to an internal reference. It provides a feedback through an optocoupler to the PWM controller IC in the primary side. The controlled current generator can be used to modify the level of current limitation by offsetting the information comi ng from the cu rrent sensing resistor. A great majority of low or medium end power supplies is voltage regulated by using shunt programmable voltage references like the TL431 (Figure 2). The galvanic insulat ion of the control information is done by using an optocoupler in linear mode with a variable photo current depending on the difference between the actual output voltage and the desired one.

2 - PRINCIPLE OF OPERATION

The current regulation loop and the voltage limitation loop use an internal 1.24V band-gap voltage reference. This voltage reference has a good precision (better than 1.5%) and e xhibits a very stable temperature behavior.

The current limitation is performed by sens i ng the voltage across the low ohmic value resistor R and comparing it to a fixed value set by the bridge composed by R

When the voltage on R on R

the output of the current loop operational

and R3 (Figure 3).

is higher than the voltage

amplifier decreases. The optocoupler current increases and t ends t o reduc e the out put voltage by the way of the PWM controller.

The voltage regulation is done by comparing a part of the output voltage (resistor bridge R

, R

and P1) to the voltage reference (1.24V). If this part is higher than 1. 24V, the output of the

voltage loop operational amplifier decreases. The optocoupler current increases and tends to

reduce the output v oltage b y the way of the P WM controller.

By enabling the TSM10 1 current source (pin 2) it is possible to offset the current sensing by a voltage equal to :

with I

= 1.4mA

OFF

R4I0⋅=

This offset lowers the output charge current and this function can be used to charge two types of batteries having different capacities. The current source is enabled by connecting pin 2 to ground.

May 2001

1/7

AN1283 - APPLICATION NOTE

3 - CALCULATION OF THE ELEMENTS

The charge current i s regulated at 700mA (if the charge control input is left open) or 200mA (if the charge control input is put to ground ), allowing the charge of two different types of batteries.

3.1 Voltage limitation

The end-of-charge voltage is limited at 1.45V/c ell, this is the recommended vol tage for an am bient

temperature at 25°C. A diode is gene rally inserted at the output of the

charger to avoid the discharge of the battery if the charger is not powered. This diode is sometimes directly integrated in the battery pack. The influence of this diode on the charge is negligible if the voltage drop (0.7V) is taken into account during the design of the charger.

The voltage at the output of the charger is :

R6R7+

OUT

and regarding R

which is a part of R6 and R7 is not considered

-------------------- -

and R7 :

-----------------------------

outVref

⋅=

ref

in this equation) The following values are used on the application

board :

❑ R

= 12kΩ

❑ R

= 1kΩ

❑ P

= 220Ω adjust for V

battery replaced by a 1kΩ resistor

❑ R ❑ C

= short circuit

= 100nF

= 15.2V with the

output

3.2 Current regulation

is the sense resistor used for current

measurement. The current regulation is effective when the

voltage drop acros s R

is equal to the voltage on

pin 5 of the T SM101 (assuming that the interna l current source is disabled).

For medium currents (<1A), a voltage drop across R

of 200mV = VR5 is a good value, R5 can be

realized with standard low c ost 0.5W resistors in parallel .

0.285Ω==

----------

(four 1.2Ω resistor in parallel) R

and R3 can be chosen using the following

formula :

–

refVR

---------------------------

⋅=

3.3 Charge control

If the pin 2 is left open, the charge current is nominal at 700mA.

If pin 2 is connected to ground, the internal current source is enabled, the current measurement is offset by a voltage equal to :

I0R4⋅=

with I

= 1.4mA

This can be used to lower the charging current or eventually to stop the charge, if V

> VR5.

In our example, the current offset is equal to 700 200mA = 500mA, representing a voltage offset V

= 140mV across R4.

The following values are used on the application board :

❑ R

= 300mΩ (four 1.2Ω-0.5W resistors in

parallel )

❑ R

= 100 Ω

❑ R

= 1.2k Ω

❑ R

= 220 Ω

❑ R

= short circuit

❑ R

= 10kΩ

❑ C

= 100nF

❑ C

= 100nF

❑ C

= output capacitor of the SMPS

❑ C

= 10µF

4 - SCHEMATIC DIAGRAM

Figure 2 represents a schematic of the output circuit of a “classical” SMPS using a TL431 for voltage regulation. This circuit is modified to use the TSM101 and the final circuit is represented in Figure 3.

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Figure 2 : SMPS Using a TL431 as Voltage Controller

Figure 3 : SMPS Using the TSM101

AN1283 - APPLICAT ION NOTE

5 - IMPROVEMENT

5.1. High frequency compensati on

Two R-C devices (R

, C2 and R10, C3) are used to

stabilize the regulation at high frequencies. The calculation of these values is not easy and is a function of the transfer function of the SMPS. A guess value for the capacitors C

and C3 is

100nF.

5.2. Power supply for TSM101

In applications requiring low voltage battery charge or when the charger is in current regulation mode, the output voltage can be too low to supply correctly the TSM101. The s ame problem occurs when the output is short-circuited. A solution to provide a quasi constant supply voltage to th e TSM101 is s hown at Fig ure 4 : an

auxiliary winding is added at the secondary side of the transformer.

This winding is forward coupled to the primary winding, the voltage across it is directly proportional to the mains rectified voltage, even if the flyback voltage is close to zero.

As this auxiliar y winding is a voltage s ource, it is necessary to add a resistor (R of the rectifier (D

A low cost regulator (Q

) to limit the current.

and Zener diode D4) is

) on the cathode

used to power the TSM101. This is necessary with autoranging SMPS with wide input voltages, for example 90 to 240V without switching. In standard SMPS with voltage range from 200 to 240V AC or 100 to 130VAC, this regulator can be removed and replaced by the small power supply shown on Figure 5 (R

aux

, C

aux

, D2).

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